HOW DO YOU design a living room that’s both unflappably calm and captivating—the decorating equivalent of Amal Clooney? New York interior designer Brian J. McCarthy faced that imbroglio with a client couple. The woman of the house craved white-on-white-on-white in their living room in the Upper West Side’s storied 1908 Apthorp building, but she didn’t speak for all. “The husband pulled me aside one day and said, ‘Brian, please give me color somewhere. Please don’t give me a vanilla box,’” recalled Mr. McCarthy, formerly a partner at Parish-Hadley, a decorating firm favored by Kennedys, Rockefellers and Gettys.
Twenty-five...